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This is a short time-lapse video of the growth of Serratia marcescens at 25 C over 20 hours. The video is time stamped every 10 minutes so students can see how the bacterial growth changes over time.

Introduction

Streak plating is a simple, reproducible method first described by Robert Koch in his seminal 1881 paper describing the importance of pure culture and methods he used to isolate bacteria (1). As such, streak plating is often the first culture technique taught in general microbiology lab courses (2). This short video can be used in general microbiology lab classes as part of a pre-lab discussion. It can be used in a microbiology lecture class dedicated to bacterial culture and isolation techniques.

Methods

In this video an individual colony of Serratia marcescens growing on Trypic Soy Agar (TSA) was subcultured onto a second plate of TSA using the quadrant streak plate method and inoculated overnight at 25 C. The images were taken using a tripod-mounted Nikon D40X being controlled with Nikon Camera Control Pro. Images were taken with a 1/8 second exposure time, without flash and an F-stop of f/5.6 and an ISO of 200. The program was set to take a picture every ten minutes over a span of 20 hours.

When complete, the images were compiled with Adobe Premiere Pro CS6 to make this short video.

Discussion

In an 1823 paper, an Italian pharmacist Bartholomeo Bizio described an organism that grew and caused bright red discoloration in food left in a warm and humid environment. Bizio named this organism Serratia marcescens after Serafino Serrati who was a famous Italian physicist and "marcescence", meaning to decay or wither (3, 4). Since then, this the bright red coloration has been used in military experiments to trace the spread of bacteria in the environment (5). It is also used in classrooms to demonstrate the growth of bacteria and the alterations of that growth by environmental changes such as mutagen exposure or temperature changes.

After approximately seven hours of growth, the Serratia begin producing red pigment that progressively appears throughout the plate over the following twelve hours. For educational purposes, the appearance of the bright colonies allows students to visualize the growth and formation of bacterial colonies from an originally invisible inoculum. This time lapse also allows students to see how the pigment first appears in areas of heavy growth and as the individual colonies enlarge, they also develop the red coloration seen from the dense growth on the left hand side of the plate. At the end of the video, students can see several individual colonies near the middle of the plate.

5. Anonymous. 1977. Hearings before the Subcommittee on Health and Scientific Research of the Committee on Human Resources. Biological testing involving human subjects by the Department of Defense, 1977. U.S. Government Printing Office, Washington, DC